OPTICAL ESR AND ELECTRICAL STUDIES OF POTASSIUM COLLOIDS IN PURE AND IMPIRITY DOPED KBR CRYSTALS

A colloidal optical absorption band peaking at 750 nm is formed in pure additively colored KBr crystals quenched from 250°C in dark. A narrow symmetrical ESR line due to colloidal centers superimposed on the well-known ESR line due to F centers in observed in crystals containing colloids. This line is Lorentzian in shape and has a half-width of 6 gauss at room temperature and a ‘g’ value of 1.9997. The presence of divalent impurities in small quantities (≤10 ppm) shifts the peak position of the colloidal band to longer wavelengths and increases the half width of the optical band and the associated CESR signal. The presence of the colloidal band gives rise to enhanced conductivity in the temperature range 250–400°C, due to thermionic emission from potassium metal. The CESR and conductivity data in impurity doped crystals show that only a small fraction of the absorption under the ‘colloidal’ band is due to colloidal particles. In crystals containing large concentrations of Cd or Zn no colloids are formed and no associated ESR signal or enhanced conductivity is observed. © 1968, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.